1. Field of the Invention
The present invention relates to a battery electrode. Especially, the present invention relates to the battery electrode capable of improving output density of a lithium ion secondary battery.
2. Description of the Related Art
Recently, from the viewpoint of environment and fuel economy, a hybrid car, an electric car, a fuel cell car are produced and sold, accompanied by continued new development of the above cars. With the above cars which are so called electric vehicles, activating of a power source capable of charging and discharging is essential. For the above power source, secondary batteries such as lithium ion battery, nickel hydrogen cell and the like, electric double layer capacitor and the like are used. Among the above, the lithium ion secondary battery, which has a high energy density and a high resistance to repeated chargings and dischargings, is considered to be preferable for use for the electric vehicle, thus leading to various on-going developments of the lithium ion secondary battery. Typically, the lithium ion secondary battery has such a structure that a positive electrode and a negative electrode are connected via an electrolyte layer, which electrodes and layer are received in a cell case. Hereinabove, the positive electrode has such a structure that a positive electrode active material layer including a positive electrode active material, a conductive additive, a binder and the like is formed on each of first and second layers of a positive electrode collector, while the negative electrode has such a structure that a negative electrode active material layer including a negative electrode active material, a conductive additive, a binder and the like is formed on each of first and second layers of a negative electrode collector.
For improving capacity density per capacity or energy density per capacity, it is preferable to form the positive electrode active material layer and the negative electrode active material layer on the collectors in such a manner that the above layers each have a thickness as great as possible. In the case of an electrode having an active material layer having a film thickness greater than or equal to 100 μm, however, a first active material in the vicinity of a surface and contacting an electrolyte layer and a second active material in the vicinity of a collector are put in conditions different in electrode reaction. Therefore, it is difficult to improve output according to thickness of the active material layer by giving sufficient performance to both of the first and second active materials.
A battery having an electrolyte layer using any of solid polymer electrolyte and gel electrolyte has such an advantage that a liquid leak from the battery is unlikely. The above electrolyte layer, however, has a high viscosity. Therefore, in the case of a thick electrode having an active material layer greater than or equal to 100 μm, the solid polymer electrolyte and gel electrolyte each are not capable of sufficiently permeating in the active material layer, thereby, as the case may be, failing to efficiently make an electrode reaction.
Therefore, a method for varying voidage of the active material layer in a stack direction is adopted.
Japanese Patent Application Laid-Open No. Heisei 9 (1997)-320569 (=JP9320569) discloses such a technology that an active material layer has a relatively low voidage in the vicinity of a collector while the active material layer has a relatively high voidage in the vicinity of a surface contacting an electrolyte layer, thus effectively permeating an electrolytic solution in the active material layer.